Production of mixed fibrous sheet material



United States Patent PRODUCTION OF emu FIBROUS SHEET MATERIAL Osborn H.Cilley and Robert L. Moore, Manheim, Pa., and Izador J. Novak, Trumbull,Conn, assignors to Raybestos-Manhattan, Inc., Passaic, N. 3., acorporationof New Jersey No Drawing. Application March 16, 1953, SerialNo. 342,724

8 Clm'rns. (Cl. 92-3) This invention relates to the production ofsmooth, strong, high temperature resistant, water-laid, felted fibroussheets composed of glass fibers and asbestos fibers, particularlysuitable for use as electrical and other insulation purposes and is acontinuation-in-part of our copending application Serial No. 283,132,filed April 18, 1952, now abandoned.

The product and process of the present invention are characterized bythe employment and ability to employ relatively long glass fibers of thetype used in glass filament yarn, in appreciable quantity, and incombination with'asbestos fibers whereby to form a composite sheet ofnovel character and wherein the respective fiber characteristicscomplement and enhance each other greatly, :as distinguished from therespective deficiencies of sheets 'felted from one of the two types offibers alone.

The present invention is further characterized and its objectsaccomplished by the employment of a colloidal dispersion of chrysotileasbestos fibers in an aqueous vehicle, the employment of which, as wehave found in accordance with the present invention, permits thedissemination therewith and suspension therein of relatively long glassfibers carrying a firmly adherent organic coating fixed on the surfacesthereof, and the production therefrom, on conventional paper makingapparatus, of composite fibrous sheets of over-all fine, smoothcharactor.

The production of colloidal dispersions of chrysotile asbestos fibers,employed herein, is more particularly described in the I. I. NovakUnited States Patent No. 2,626,213, said patent also describing theproduction of water-laid felted fibrous asbestos sheets therefrom andthe novel properties of such dried residues or sheets. Although suchsheets are much stronger and tougher than conventional asbestos paper,they are relatively soft, weak, and densely packed as compared to glassfiber sheets which are relatively harsh, strong, and porous. Glassfibers are comparatively large, of great strength, but fragile toabrasion against each other; whereas asbestos fibers are much smaller,relatively weak, but not fragile. When admixed, the properties of thesetwo fibers complement each other: the cushioning effect of the softer,weaker, much finer asbestos fiber between the hard incompressible glassfibers prevents them from injuring each other and fills the intersticesto give the product an over-all fine, smooth structure.

Furthermore, one of the serious deficiencies of an allglass fiber sheetis that on exposure to heat above the melting point of the glass, sayabove 1500" F., it fuses to globules which have lost all resemblance toa sheet; whereas the combination of glass fiber and asbestos, while itbecomes more brittle, does not melt down even when the glass content is80%, but shrinks somewhat and retains its sheet form without blistering,crumbling, or distortion. In electrical insulation this quality oflargely retaining its space factor, or its spacing apart of insulatedconductors under such destructive conditions, is an extreme- "ice 1yvaluable property of high temperature resistant insulation. In most hightemperature electrical insulation uses, this quality spells thedifierence between continued insulationand short circuiting, whenexposed to flame. An all asbestos sheet has the property of retainingits space factor, but it was not predictable that an asbestos sheetcontaining a large proportion of glass fiber would also stand up so wellunder flame temperatures.

By the present invention, a smooth, heat-resistant sheet is obtained,wherein great strength is derived from the glass component, and acushioned structure from the asbestos component. This set of propertiescauses this combination sheet to be of greater value for laminations,electrical insulation, and heat resistant uses, than either theall-glass or all-asbestos sheet by itself. Good strength is of greatimportance in treating thin sheet material because of the tensionrequired to keep such'sheets free from wrinkles during wetting anddrying processes.

The presence of the coated glass fibers improves the paper makingproperties of the colloidal dispersion of asbestos fiber by providing afreer, less plastic wet web than with colloidally dispersed asbestosalone; and, conversely, the viscous properties of the colloidaldispersion of asbestos make it possible'to maintain the fine, coatedglass fibers individualized'and uniformly distributed in the papermakingslurry of'the combined fibers. The coated glass fibers are, ofcourse, not colloidally dispersed, being much too large, their diameterbeing ordinarily 5 to 10 microns. Even the finest available glass fiberat .75 micron diameter is above the colloidal range of size. Asbestosfibers in which the unit fibers are 1 to micron are well Within thecolloidal range and a colloidal dispersion of them can be made and isemployed here.

We find that it is necessary for the practice of the present inventionto use coated glass fibers; that is, glass fibers carrying a wettable,firmly adhered, water insoluble, hardened organic coating which,although it may be very thin, adheres tightly to the ordinarily highlyhydrophilic surface of the-glass fibers throughout the mixing and papermaking operations in the presence of colloidal asbestos and colloidizingagents. Uncoated glass fibers, or glass fibers carrying a temporary andwater removable coating which, because of solubility in the liquidmedium or'poor resistance to abrasion during mixing, for example, oilyor greasy'lubricants, leaves the glass surface unprotected; donot'disp'e'rse in the asbestos colloid and clot undesirably and do'notallow the formation of a smooth glass slurry, asdo glass fibers with theindicated proper coating. A film tacky when wet, or one nonwettable bywater, or detergent-containing water, is also undesirable.

Satisfactory coatings are those which are known for application to glassfiber later to be used to provide good adherence to plastics such asphenolic, urea, melamine, polyester, acrylic, and the like resins. Thesecoatings may be of the strongly glass surface adherent types such asthose containing suitable Werner complexes, for example, a type ofWerner complex compound based on an atom selected from the groupconsisting of chromium, cobalt, nickel, copper and lead havingcoordinated therewith an acido group having 1 to 6 carbon atoms andcontaining a labile hydrogen group, and other bonding agents asdescribed in Steinman U. S. Patent No. 2,552,910. EX- arnple 3 of saidpatent specifies a type of coating composition which we have foundsatisfactory. Others are butylated urea resin, water soluble ureaformaldehyde, or melamine formaldehyde resin wherethe coating is bakedon to harden and insolubilize it in water. Proteins of the type ofgelatin and albumin may also be employed, but these must be set, e. g.,with formaldehyde or chromates.

Additional examples of satisfactory coating compounds strongly adherentto glass are the Werner type chromium compounds described in UnitedStates Patents 2,273,040; 2,356,161; 2,544,666; 2,544,667; 2,544,668;2,552,910; and 2,611,718; hydrolyzed ethyl silicate and vinyl acetateresin described in U. S. Patent 2,215,048; alkyd resins as described inU. 8. Patent 2,258,708; polyvinyl butyral asdescribed in U. S. Patent2,354,110;,phenyl ethyl solibone as described in U. S. Patent 2,371,050;organic polysiloxane. as described in U. S. Patent 2,392,805; ureaformaldehyde, melaminev formaldehyde and alkyd resin as described in U.S. Patent 2,397,242; resorcinol formaldehyde resin as described in U. S.Patent 2,403,872; heat treated dextrinized starch as described in U. S.Patent 2,446,119; protein-formaldehyde (gelatin, blood, albumen, casein)as described in U. S. Patent 2,477,407; allyl silicates as described inU. S. Patent 2,513,268; vorgano silicon halides followed by resincoating as described in U. S. Patent 2,557,786; diallyl diethoxy silaneand other silanes as describedin U. S. Patent 2,563,288; and polyvinylacetal V as describe din U. S. Patent 2,572,407. All these substancesapplied to the glass meet the requirements of strong adhesion, waterinsolubility and freedom from tack while glet, which are the essentialsfor a satisfactory protective m. e It should be noted that even withthese coated glass fibers,'they do not disperse to individuals in plainor detergent-containing water, and also do not disperse in a slurry ofasbestos fibers containing no colloidizing agent. The asbestos must becolloidized to suspend these precoated glass fibers as individuals, andthis combination is necessary for our purpose. Glass fibers hereinmentioned is intended to mean the coated glass fibers above menfioned. VV In preparing the aqueous slurry of asbestos and coated glass fibers,We may combine the two in any order. For

. tion, then added to the requisite amount of water containing asbestoscolloidizing agent, followed by agitation to colloidally disperse theasbestos fibers and disseminate and suspend the coated glass fiberstherein. Another procedure is to first prepare a heavy slurry ofasbestos fibers water containing the asbestos colloidizing agent, thenadd the coated glass fiber, and thereafter add more water and agitatethe whole. In a preferred procedure we firs t prepare the colloidaldispersion of the asbestos fibers and then add the coated glass fiberswith suitable stirring to individualize the coated glass fibers and tothoroughly disseminate them throughout and suspend them in the'colloidaldispersion of the asbestos fibers.

As more particularly described in the aforesaid Novak Patent No.2,626,213, a [colloidal dispersion of asbestos fibers may beprep'aredfrom chrysotile asbestos agglomerates, such as,for example, mill fiberof long or spinning grade length (bagged asbestos fiber of commerce), byplacing same in a vessel equipped with vertical cylindrical mixing bars,and gradually mixed therein With' water containing chrysotile asbestoscolloiding agent to form a slurry of desired concentrationforsubsequen-t sheeting on'a paper making machine. The amount ofcolloidizing agent employed varies with the character of the particularagent, the amount of asbestos, and the concentration of the slurry. Thusthe aqueous solution should contain an amount of colloidizing agentfirst efiective to subdivide the asbestos agglomerates into fibrils ofcolloidal size and to saturate the fibril surfaces with and to depositthereon an adsorbed layer of said agent; and second to colloidallydisperse said'saturated fibrils, this being accomplished by employing anamount of agent in excess of that adsorbable on the asbestos andproviding'an added or free colloidal dispersion forming and maintainingincrement, whereby 'a stable colloidal dispersion of fine fibers isformed, the majority having a diameter of between 200 and 500 angstromunits; These colloidal dispersions are of a gelatinous, viscouscharacter, the viscosity increasing with increase in colloidal fibercontent or concentration.

For the purpose of insuring stability of the colloidal dispersion insubsequent handling, the amount of c01- loidizing agent added isgenerally in excess of the minimum amount required to maintain thecolloidal dispersion above its break point, or the concentration of theslurry at which more water, without addition of agent, would cause thecolloid to break, the asbestos fibers to clot, and the fibers becomeagain agglomerated. This excess agent may befrom about 10% to 100% ormore, andin addition to insuring stability of the colloidal dispersion,increasing amounts tend to increase the viscosity of the col= loidaldispersion. This is desirable 'in the practice ofthe present inventionfor the purpose of aiding in suspending increasingly larger proportionsof coated glass fibers. The admixture of individualized coated glassfibers with a colloidaldispersion of asbestos fibers further increasesthe viscosity of the resultant mixture.

The coated glass fibers employed are those of the readily availablepresent commercial range, having a diameter below 10 microns, andpreferably 6 to 10 microns, although fiber of somewhat larger diametermay be used if coarseness of the paper'is not critical, and smaller 7diameter, down to 4 micron, will give a correspondingly.

finer structure. A suitable commercial source of glass fiber iscontinuous filament glass yarn composed of strands each comprised of 204filaments of about 6 microns in diameter. The coating of the glassfilaments is usually applied during the process of making the yarn,between the filament-forming stage and the condensation of the filamentsinto the yarn. The yarn is chopped into lengths of from about /s inch toabout one inch, and preferably about Me inch. This cut glass yarn is tobe distinguished from ordinary mineral or slag wool which is toodefective in glass quality, too impure, and too coarse for the practiceof the present invention, and does not, unless intentionally done, carrya coating of the required type.

The amount of coated glass fiber which can be incorporated and stablydistributed with a colloidal dispersion of asbestos fibers, aspreviously indicated, is to some extent dependent upon the viscosity ofthe colloidal asbestos dispersion and that of the final coated glassfiber: colloidal asbestos dispersion, and should thus be correlatedthereto. The addition of the coated glass fiber to the asbestosdispersion increases the viscosity; the more glass, the higher theviscosity of the combination. If the viscosity of a glass-asbestosslurry drops much below twice that of water, the glass fibers tend toform clots of considerable size and the uniformity of the slurry isspoiled. The'more glass in the slurry the higher the viscosity neededfor safe storage, and the shorter the safe period of stirringsbelow thestable viscosity before the glass fiber beginsto ball up and separateout. By controlling the viscosity, we can form mixes composed of fromabout 1% to about 75% of coated glass fibers, and preferably from about10% to about of glass of the total weight of the fibrous components. V

The mixing of the chopped strands of coated glass fibers with thecolloidal dispersion of asbestos fibers serves to open up the strandsand individualize the coated glass fibers and to uniformly distributethem with the asbestos fibers and stably suspend them in the colloidaldispersion.

The following specific examples are illustrative of the presentinvention: 7

Example I A colloidal dispersion of chrysotile asbestos fibers in Theforegoing dispersion at the indicated dilution and with the indicateddispersing a'gent could have'been pre-' pared with approximately 25% ofsodium oleate by weight of the asbestos, and thus the agent is 100% inexcess. Upon mixing coated glass fibers, of the character previouslydescribed, with the colloidal asbestos dispersion to form asubstantially homogeneous mixture, it Was found that it could readilysupport up to 100% of coated glass fibers by weight of the asbestosfibers. At this total fiber concentration of .75% the slurry had a finalviscosity of five times that of water, and the slurry was storage stableand the glass fibers remained suspended in an individualized manner.

Example 11 A colloidal dispersion of chrysotile asbestos of #3 grade,Canadian standard (spinning grade length) was prepared as follows: In7,500 lbs. water there was combined 150 lbs. ammonium oleate as theasbestos dispers ing agent, and 200 lbs. 3 K Canadian asbestos fiber,with stirring until the mixture was uniform, whereupon 100 lbs. /2 longchopped strand, cut from glass yarn, was added. The glass yarn carried acoating of the type of Example 3 of U. S. Patent 2,552,910. Each strandcomprised 204 filaments about 6 microns in diameter. The heavy slurry(approximately 4% solids) was then mixed until both the asbestos andglass fiber were thoroughly distributed and intermingled. The slurry wasthen diluted by addition of water to 1% solids, and passed throughslotted screens and centrifugal separators to remove foreign and oversze material. There was no selective separation of the glass fiber, inspite of its relatively large size, compared with asbestos fiber. Thisfurther indicated the uniform quality of the mix. The slurry was then atpi l 9, which was then raised to pH by the addition of KOH, a procedurewhich improves the freeneSs and reduces the adhesiveness of colloidalasbestos dispersions of the water-soluble fatty acid soap type withoutdamage to the dispersions, as more particularly described and claimed inthe copending application of I. J. Novak, Serial No. 279,951, filedApril 1, 1952, now Patent No. 2,652,325. The viscosity of the slurry wasabout 2 /2 times that of water.

The prepared slurry was then run oil on a tissue type Fourdrinier papermachine equipped with sprays of dilute (.1%) soap water to avoidbreaking the asbestos dispersion. Thicknesses from 1 mil to 10 mils weremade, all sheets bein smooth, firm, well formed, and of very uniformglass fiber distribution. Without very close examination it was notpossible to distinguish the glass fiber content. it contained about 40%of glass fiber, .since more asbestos passes through the Fourdrinier wireduring formation than the longer fibered glass. The 10 mil sheet whichweighed 8 grams per square foot, however, was considerably stronger andtougher than equivalent weight asbestos sheets, and felt almost lfire athin suede leather. It broke when fresh at two pounds per inch of width.After one Week, it broke at nine pounds per inch of width. Vt/hen heattreated one minute at 1350 F., which burns off the organic matter leftfrom the soap treatment, the 10 mil sheet had a breaking strength of tenpounds per inch of width. It had good wet strength, both in water and insolvents. On heating in a Bunsen burner blue flame at a red heat forseveral minutes, it did not'blister, crumble, deveiop holes or collapse,but retained its sheet form, with only slight shrinkage. The heattreatment may be used to improve wet strength, or a starch, glue, orother treatment may be applied to the original sheet, depending on thesubsequent use.

Example III One hundred pounds of #3 F (spinning grade) Canadianchrysotile asbestos fiber was added to 300 pounds of a solution ofdiocetyl sodium sulfosuccinate (Aerosol O. T.) as the asbestoscolloidizing agent; 2175 pounds of Water was further added and mixedforfour hours'to form a 4% colloidal asbestos dispersion. This was furtherdiluted with water to a 0.50% asbestos content, andpassed through papertype slotted screens and centrifugal vortex separators to removeimpurities such as sand and unopenable splinty asbestos material andthen over mag netic rolls to remove residual particles of magnetite.While mixing, pounds of /2-inch long cut strand from glass yarn (204filaments per strand, each 6 microns in diameter) was added and mixedfor one-half hour. The glass. yarn was of the type commonly used forpolyester impregnation carrying a coating of the Werner type chromiumcompound previously referred to (Owens Corning #863). This wasrescreened to remove overlength glass strands which occur in the cutstrands of glass yarn. At this point the dispersion had a fiberconcentration of glass plus asbestos of l.\i0%, a viscosity five timesthat of water, was of a smooth character with no glass fibers showing,the glass filaments having become interspersed with the colloidalasbestos fibers in a substantially homogeneous mixture, and the glassfibers remained suspended by reason of the viscosity of the dispersionand by reason of suspension by and entrainment with the colloidallydispersed asbestos. Uniformity of asbestos and glass dispersion wasevidenced by the high shimmer of the slurry while in motion.

This slurry was sheeted out as previously, on a paper making machine,into paper of about 10 mils thickness and a weight of .015 pound persquare foot. The glass content of this sheet was 60%.

In the foregoing example, the colloidal dispersion of asbestos alone atthe indicated dilution could have been prepared with l52i)% of AerosolO. T. by weight of the asbestos, and ordinarily this amount is doubledfor safety in handling storage. Since in the present example 100 partsof asbestos to parts of glass was employed, added suspending agent forthe coated glass fibers is desirable, and this is accomplished byemploying a greater excess'of Aerosol O. T., it having been found thatthe added'agent improves the stability of the colloidal asbestosdispersion to mechanical abrasion by the very much larger diameter (300to 1) glass fibers. ()therwise this mechanical abrasion might reduce theasbestos fiber to much shorter lengths and would result in lowerviscosity for the col loidal asbestos dispersion and thereby allow thecoated glass fibers to become aggregated or clotted. Control byrestricting the dilution to hold the viscosity above twice that ofWater, and preferably higher, is also effective even when minimumdispersing agent is used. Viscositi'es up to eight times that of Waterwill make smooth paper, be cause of the uniform, unclotted character ofthe colloidal asbestos dispersion and the uniform glass distribution.This is particularly desirable for heavy papers. For thin papers:

or tissues a lower solids content and viscosity must be used to get goodfiber distribution in the tissue.

The proportion of the coated glass fiber of either of the foregoingexamples in clear water, upon agitation, re-

sulted only in shortening of the filaments and entangling clotting, andcould not be made into paper. Even the use of this coated glass fiberalone in the above dilutions" but containing the full indicated amountsof asbestos col loidizing agent did not produce an individualizedsuspension of the glass fibers, and they were still clotted andentangled on mixing, and rapidly settled out. It was there-: foreunexpected that the coated glass fibers would remain: separated,suspended, individualized, and free from clots in: combination with acolloidal dispersion of asbestos fibers-I in accordance with the presentinvention.

Although it is convenient to add a suitable excess of asbestoscoilodizing agent beyond that necessary to pro vide safety in handlingand storage of the colloidal asbestos dispersion, for the purpose ofproviding an added glass: fiber suspending component, and even thoughthe usual excess is generally adequate for suspending appreciably largeproportions of glass fiber, we may add other vis- .7 V cosity builders,such as polyvinyl alcohol, methyl cellulose, carboxy methyl cellulose,alginates, and other water soluble thickening agents, instead of excesscollodizing agent to act as supplemental suspending agents. Thus, thenecessary viscosity may be obtained by concentration of the asbestos:glass slurry, by selection of asbestos fiber types or lengths which givediiferent viscosities; e. g., for any given concentration the viscosityof a colloidal asbestos increases with fiber length, by the addition ofviscosity builders or other means such as more or less fiber opening,temperature, etc.

It will of course be understood that other asbestos colloidizing agentsmay be employed according to the methods and procedures described in theaforesaid Novak Patent 2,626,213.

Thesheet material of the present invention accepts impregnation readilyand uniformly. Such use, for example, may be for phenol resin ormelamine resin bonded laminates in sheet, rod, or tube form. Whenimpregnated with vinyl resins and slit into tape Width, the sheetmaterial of the present invention will provide a superior typeinsulating tape for the insulation of heat and flame-resistant wires andcables. When coated with silicone resins, silastic rubbers orpolyfluorocarbons, the sheet will provide a superior type heat-resistinginsulation for slot liners and separators in the winding of heavy dutyelectric motors.

Other uses for the sheet material of the present inven' tion are such asfor slitting into tapes and spinning into yarn in accordance with themethod described in the Novak-Palm United States Patent No. 2,578,941.Thus a sheet formed in accordance with the present invention andcomposed of the dried residue of a colloidal dispersion of spinninggrade chrysotile asbestos fibers and containing 10% by weight of glassfibers of the character previously described, and having a weight of .55gram per square foot, was slit into tapes and spun into a 50 cut yarn inaccordance with the aforesaid patent. It exhibited a tensile strengthfor a single strand of 1.54 pounds as compared to 1.28 for a likeasbestos sheet without glass fiber.

.. We claim as our invention:

' 1. A smooth Water-laid felted fibrous sheet comprised of the driedresidue of an alkaline colloidal dispersion of chrysotile asbestosfibers, said sheet including individualized cut glass yarn fiberssubstantially uniformly distributed therethrough, the glass fiberscarrying a wettable, water-insoluble, strongly adherent hardened organicfilm coating.

2. A smooth water-laid felted fibrous sheet comprised of the driedresidue of an alkaline colloidal dispersion of chrysotile asbestosfibers of spinning grade length, said sheet including individualizedrelatively long cut glass yarn fibers substantially uniformlydistributed therethrough, the glass fibers carrying a wettable,water-insoluble, strongly adherent hardened organic film coating.

3. A smooth water-laid felted fibrous sheet comprised of the driedresidue of an alkaline colloidal dispersion of chrysotile asbestosfibers, said sheet including from about 1% to about 75% by weight ofindividualized cut glass yarn fibers of from about A; to about 1 inch inlength and less than about 10 microns in diameter substantiallyuniformly distributed therethrough, the glass fibers carrying a'wettable, Water-insoluble, strongly adherent hardened organic filmcoating.

4. A smooth water-laid felted fibrous sheet comprised of the driedresidue of an alkaline colloidal dispersion of chrysotile asbestosfibers, said sheet including from about 10% to about 75% by weight ofindividualized cut glass yarn fibers of from about to about 1 inch inlength and from about 5 to about microns in diameter substantiallyuniformly distributed therethrough, the glass fibers carrying a'wettable, water-insoluble, strongly adherent hardened organic filmcoating.

5. The method of forming a smooth sheet composed of a substantiallyhomogeneous mixture of asbestos and ing organic chrysotile asbestoscolloidizing agent to uniformly distribute the fibers with each other,and then filtering the aqueous liquid from the mixture.

6. The method of forming a smooth sheet composed of a substantiallyhomogeneous mixture of asbestos and glass fibers, which comprises mixingglass filamentsof from about 5 to about 10 microns in diameter and offrom about /8 to about 1 inch in length derived from nut glass yarn andcarrying a wettable, water-insoluble, strongly adherent hardened organicfilm coating, with an aqueous alkaline colloidal dispersion ofchrysotile asbestos fibers of spinning grade length containing organicchrysotile asbestos colloidizing agent to form a slurry having aviscosity at least twice that of water and to thereby stably suspend theglass filaments in the colloidal asbestos dispersion, and then filteringthe aqueous liquid from the mixture.

7. The method of forming a smooth strong sheet composed of asubstantially homogeneous mixture of asbestos fibers and from about 1%to about of glass fibers by Weight of the composite, which comprisesmixing'glass filaments of less than about 10 microns in diameter andfrom about A; to about 1 inch in length derived from cut glass yarn andcarrying a wettable, Water-insoluble, strong adherent hardened organicfilm coating, With an aqueous alkaline colloidal dispersion ofchrysotile asbestos fibers containing organic chrysotile asbestoscolloidizing agent to form a slurry having a viscosity of from abouttwice to about eight timesth at of Water to thereby stably suspend saidglass filaments therein and to uniformly distribute the fibers With eachother in an individualized manner, and then felting said fibers on aforaminous carrier of a paper making machine.

8. A stable substantially homogeneous paper making slurry comprised ofan aqueous alkaline colloidal dispersion of chrysotile asbestos fibersincluding from about 1% to about 75 of glass fibers by weight of thetotal fibers, said glass fibers being from about /8 inch to about 1 inchin length and from about 5 to about 10 microns in diameter derived fromcut glass yarn, and substantially uniformly distributed through andstably suspended in an individualized manner in said colloidaldispersion of asbestos fibers, the glass fibers carrying a wettable,water-' insoluble, strongly adherent hardened organic film coating.

References Cited in the file of this patent UNITED STATES PATENTSMoeller Dec. 1, 1914

1. A SMOOTH WATER-LAID FELTED FIBROUS SHEET COMPRISING OF THE DRIEDRESIDUE OF AN ALKALINE COLLOIDAL DISPERSION OF CHRYSOTILE ASBESTOSFIBERS, SAID SHEET INCLUDING INDIVIDUALIZED CUT GLASS YARN FIBERSSUBSTANTIALLY UNIFORMLY DISTRIBUTED THERETHROUGH, THE GLASS FIBERSCARRYING A WETTABLE, WATER-INSOLUBLE, STRONGLY ADHERENT HARDENED ORGANICFILM COATING.
 7. THE METHOD OF FORMING A SMOOTH STRONG SHEET COMPOSED OFA SUBSTANTIALLY HOMOGENEOUS MIXTURE OF ASBESTOS FIBERS AND FROM ABOUT 1%TO ABOUT 75% OF GLASS FIBERS BY WEIGHT OF THE COMPOSITE, WHICH COMPRISESMIXING GLASS FILAMENTS OF LESS THAN ABOUT 10 MICRONS IN DIAMETER ANDFROM ABOUT 1/8 TO ABOUT 1 INCH IN LENGTH DERIVED FROM CUT GLASS YARN ANDCARRYING A WETTABLE, WATER-INSOLUBLE, STRONG ADHERENT HARDENING ORGANICFILM COATING, WHILE AN AQUEOUS ALKALINE COLLOIDAL DISPERSION OFCHRYSOTILE ASBESTOS FIBERS CONTAINING ORGANIC CHRYSOTILE ASBESTOSCOLLOIDIZING AGENT TO FORM A SLURRY HAVING A VISCOSITY OF FROM ABOUTTWICE TO ABOUT EIGHT TIMES THAT OF WATER TO THEREBY STABLY SUSPEND SAIDGLASS FILAMENTS THEREIN AND TO UNIFORMLY DISTRIBUTE THE FIBERS WITH EACHOTHER IN AN INDIVIDUALIZED MANNER, AND THEN FELTING SAID FIBERS ON AFORAMINOUS CARRIER OF A PAPER MAKING MACHINE.